Paper is generally prepared by producing a slurry of cellulosic fibers in water fortified with a variety of additives and then removing most of the water to form a thin paper web. The structural integrity of the paper arises, in large part, from mechanical entanglement of the cellulosic fibers in the web and hydrogen bonding between fibers.
With paper intended for use as tissue and towel products, i.e., facial tissue, bathroom tissue, paper towels, napkins and the like, the level of structural integrity arising from the paper-making process conflicts somewhat with the degree of perceived softness that is necessary for consumer acceptance of such products.
The most common method of increasing the perceived softness of tissue and towel products is to crepe the paper. Creping is a means of mechanically compacting paper in the machine direction. The creping action imparts a fine, rippled texture to the sheet and also increases the bulk of the sheet, resulting in improved softness and absorbency, as well as other significant changes in many physical properties such as stretch, particularly when measured in the machine direction.
Creping is generally accomplished by fixing the cellulosic, paper web to a thermal drum dryer, generally called a Yankee dryer, by applying the web onto the surface of the dryer onto which has been sprayed a combination of an adhesive and a release agent, generally in the form of an aqueous solution, emulsion or dispersion. The web then is scraped backwardly upon itself and off of the Yankee dryer by means of a flexible blade, a so-called doctor blade. This blade is also sometimes referred to as a creping blade or simply a creper.
A Yankee dryer is a large diameter cylinder, generally an 8-20 foot diameter drum. Typically, the drum is heated with high pressure steam, providing a hot surface that completes drying the paper web at the end of the papermaking process. Prior to encountering the Yankee dryer, the paper web, formed by dewatering the cellulosic fiber slurry, is often transferred to a felt or fabric in a so-called press section where de-watering is continued to increase the consistency of the paper, usually to about 40 to 80%, either by mechanically compacting the paper or by some other de-watering method such as through-air-drying with hot air before feeding the Yankee dryer. Thereafter, the paper web is transferred in this partially dry, high solids condition to the surface of the Yankee dryer.
The process of creping causes a significant number of inter-fiber bonds to break, which alters many characteristics of the web and increases the perceived softness of the resulting tissue and towel products.
While some amount of adhesive build-up on the dryer surface is needed to control the creping (adhesion) process, excessive build-up of adhesive or streaks of adhesive on the dryer surface, which can occur with some types of adhesives, interferes with the creping process. Streaks can cause differences in the profile of adhesion across the width of the dryer and can result in humps or wrinkles in the finished roll of paper. Such build-up also may cause the creping blade to ride on the coating, causing blade chatter and reducing blade life. Sometimes a second doctor blade is positioned after the creping blade in order to remove excess creping adhesive and other residue.
Obtaining and maintaining the proper level of adhesion between the paper web and the Yankee dryer thus is an important factor in determining crepe quality. Generally, inadequate adhesion results in poor or non-existing creping, while excessive adhesion may result in poor sheet quality and interferes with stable operation. For example, webs which are insufficiently adhered to the Yankee dryer impact the control of the web as it travels in the space between the creping blade and the winder upon which a roll of the paper is being formed, causing problems in forming a uniform roll of paper. In particular, a loose sheet between the creper and the roll can cause wrinkles, foldovers, or weaving of the edges of the sheet in the rolled-up paper. Poorly formed rolls not only affect the reliability of the papermaking operation, but also the subsequent operations of tissue and towel manufacture in which the rolls are converted into the tissue and paper towel products.
The level of adhesion between the paper web and the Yankee dryer also impacts the efficiency at which the web is dried. Higher levels of adhesion reduce the impedance of heat transfer and cause the web to dry faster, enabling more energy efficient, higher speed operation, while poor adhesion interferes with the high processing speeds needed in today's papermaking operations.
Conventionally, a creping adhesive, alone or in combination with a release agent, is applied by spraying onto the surface of the Yankee dryer in order to provide the appropriate adhesion between the paper web and the dryer surface to produce the desired degree of crepe.
Over the years, various types of creping adhesives have been used to adhere the paper web to the dryer surface, i.e., to the surface of the Yankee dryer. Some examples of prior art creping adhesives are disclosed in U.S. Pat. Nos. 4,886,579; 4,528,316 and 4,501,640. Historically, one of the most common of the classes of polymers used as creping adhesives has been the thermosetting polyaminoamide-epichlorohydrin polymers (PAE polymers). Polyamidoamine-epichlorohydrin resins have been used as creping adhesives in the following patents: U.S. Pat. No. 5,338,807; U.S. Pat. No. 5,786,429; U.S. Pat. No. 5,902,862 and U.S. Pat. No. 5,944,954. These adhesives were designed to provide adhesion and good performance under high moisture conditions.
As known to those skilled in the art, different creped products, however, require different levels of adhesion, tack and rewetability for producing a product with consistent characteristics. For example a web destined for use as facial tissue will require a different level of adhesion, tack and rewetability compared to a web destined for making paper towels. Rewetability refers to the ability of moisture contained in the partially dried paper web to enhance the level of tack of the adhesive remaining on the dryer surface when the web is brought into contact with the heated dryer. A marked increase in tack is indicative of high rewetability. The rewetability of the adhesive is important because only a portion of the surface of the dryer is normally covered with a new application of adhesive on a given rotation of the Yankee dryer.
One problem that has confronted thermosetting polyamide-epichlorohydrin polymers when used as a creping adhesive is that polymer properties, including rewetability and the level of adhesion, are to a large extent restricted by the degree of epichlorohydrin cross-linking. This makes it difficult to vary polymer properties at the time the adhesive is being used in any given creping process. When conventional PAE-type adhesives encounter low moisture/high temperature conditions the residual adhesive on the dryer surface tends to become hard and brittle. This change in properties causes both an increase in adhesive build-up on the dryer surface and a decrease in adhesion (tack), leading to many of the problems outlined above.
Paper makers also have long recognized that different creping processes, different creping machines and paper webs based on different wood pulps have different adhesive demands if one is to optimize the creping process. Having an ability to control and change the amount of cross-linking to achieve an optimum level of adhesion for creping in a given papermaking process is a desired goal. Unfortunately, with prior art thermosetting polyamide-epichlorohydrin (PAE-type) polymers that degree of control has not been possible because the cross-linking of the adhesive occurs during the manufacturing process and it is impractical to have a number of water-soluble, thermosetting cationic polyamide-epichlorohydrin (PAE-type) polymers with different degrees of cross-linking in an attempt to tailor the creping adhesive to the creping process.
The prior art has attempted to remedy the shortcomings of the PAE-type polymers by altering the adhesive properties of the polymer with the addition of a release aid. A release aid provides lubrication to the doctor blade, moderates the adhesive properties of the adhesive coating to influence the release of the paper web from the Yankee dryer, and improves the absorbency of the paper product. Some release aids are non-polar, neutral and non-water soluble compounds. Cationic release aids are also available and comprise water insoluble materials, such as imidazoline quaternary salts.
Notwithstanding prior art efforts to improve the creping process, a need still exists for ways of managing the creping process by allowing more control over the level of adhesion and adhesive rewetability and thus the degree of adhesion between the tissue or towel substrate and the surface of the dryer from which the substrate is creped. A creping adhesive that provides a high level of adhesion to the Yankee drum under varying moisture conditions and particularly under low moisture conditions, while also providing high levels of water resistance for acceptable sheet uniformity and tissue quality, would be highly desirable.
The present invention focuses on an improved adhesive composition based on PAE-type polymers. In particular, the present invention provides a PAE-type adhesive for Groping and a related process for creping in which the character of the adhesive can be adapted and varied to control the adhesion level and thus better optimize a given creping process.